Employee portals are utilized by many companies to improve companies’ information exchange, communication, and employee collaboration, as well as to better support their business processes. Owing to limited IT budgets and the need to justify investments in employee portals, assessing the benefits of these is an important field in research and practice. Thus, the purpose of this study is to gain a better understanding of employee portal success. We introduce a theoretical model for this that is based on the DeLone and McLean IS Success Model. Furthermore, we develop hypotheses regarding the associations between different models’ success dimensions and test them using more than 10,000 employees’ responses collected in a survey of 22 companies. Our results indicate that besides the factors contributing to IS success in general, other success dimensions – like the quality of the collaboration and process support – have to be considered when aiming for a successful employee portal. The study’s findings make it possible for practitioners to understand the levers with which to improve their employee portals. By empirically validating a comprehensive success model for employee portals, the study’s results advance theoretical development in the area of collaboration-centered systems and present a basis for further research in this field.